An optical lens (hereinafter referred simply to lens) for various optical apparatuses, e.g., projection type display apparatus, sometimes referred to LCD projector (so-called a rear or front projector, hereinafter referred simply to a projector), digital camera, video camera, CCD camera and optical microscope, comprises a base made of a highly transparent material, e.g., glass or acrylic resin.
A lens is coated with an anti-reflective membrane, when used for a display apparatus, e.g., LCD projector, where light reflected from the lens significantly affects image formation, in order to prevent adverse effect by reflected light on the optical system in the apparatus. The common anti-reflective membrane has a mono- or multi-layered structure.
A mono-layered anti-reflective membrane is made of magnesium fluoride in most situations. It has a refractive index of 1.38, which is considerably lower than that of glass (refractive index: 1.5 to 1.54) or acrylic resin (refractive index: 1.49). Thickness of an anti-reflective membrane is determined by dividing light wavelength in a range from 540 to 550 nm, which is considered to be that of the visible light which human eyes perceive most sensitively, by four times of refractive index of the membrane material. More specifically, the thickness is set at around 100 nm as a target, when magnesium fluoride is used.
For a multi-layered membrane, two to five layers are laminated to decrease refractive index. For example, a three-layered anti-reflective membrane to be formed on glass (refractive index: 1.52, for example) may comprise 3 materials of different refractive index, slightly higher, considerably higher and considerably lower than that of glass, in this order from the glass. One example of this structure comprises cesium fluoride (refractive index: 1.63), zirconium oxide (refractive index: 2.10) and magnesium fluoride (refractive index: 1.38), in this order from the glass. A multi-layered membrane has a more accurately controlled thickness, because it is produced by a vacuum process, e.g., sputtering or deposition. Magnesium fluoride is a normal choice for the outermost layer, because it is considerably lower in refractive index than glass and highly transparent, and can be formed into a membrane by vacuum deposition.
By the way, magnesium fluoride involves problems of being stained with dirt and dust in air (e.g., tar, soot and the like) to lose its light transmittance. Some of the major causes for greatly decreasing lens transmittance include oil mist in beef barbecue restaurants, and tar and smoke in confined spaces, which darken or blur displayed images. It is difficult for an inexpert to clean a stained optical system in an apparatus, especially in projector. Therefore, an optical system with a stained lens may have to be replaced totally at worst.
One of the methods proposed to protect the lens surface from staining is coating the surface with a fluorine-based resin, known for its high liquid repellency, to decrease surface energy. A coating membrane of fluorine-based resin has a higher contact angle with water than a magnesium fluoride membrane, around 100 to 110° versus 40 to 50°, and has a higher effect of preventing surface staining. However, a fluorine-based resin has a high electrical resistance. For example, fluorine-based resins, beginning with polytetrafluoroethylene, have a much higher surface resistance than glass, as high as 1016 to 1018Ω versus 101l to 1012Ω. Therefore, they tend to be charged with built-up static electricity, causing problems of statically attracting dust or the like, which is more noted in low humidity conditions in winter.
An anti-reflective membrane may lose its anti-reflective function, when coated with an excessively thick membrane. A mono-layered anti-reflective membrane, for example, is designed to have a thickness of λ/4n, where λ: wavelength of the visible light which human eyes perceive most sensitively, and n: refractive index of the membrane material. A membrane having a thickness deviating from the above level will have a deteriorated effect of decreasing reflectivity. For example, an anti-reflective membrane of magnesium fluoride, which has a refractive index of 1.38, is produced to have a target thickness of 94 to 100 nm. Coating the anti-reflective membrane with a liquid repellent layer, e.g., of silicon-based material having a higher refractive index than magnesium fluoride, increases its reflectivity, even when the layer has a thickness of 10 nm or so. More specifically, coating the anti-reflective membrane with a liquid repellent layer of silicon-based material having a refractive index of 1.45 to an average thickness of 10 nm increases its reflectivity at a wavelength of 550 nm to around 3% from 0.8% as that of the uncoated one.
Moreover, the anti-reflective membrane will have a deteriorated anti-reflective function, even when coated with a fluorine-based resin having a lower refractive index (e.g., 1.34) than magnesium fluoride to a certain thickness. The inventors of the present invention have found that the anti-reflective membrane tends to have an increased reflectivity, when coated with the resin to a thickness of above 20 nm, although keeps its reflectivity essentially unchanged irrespective of thickness of the coating membrane so long as it is 20 nm or less. It is therefore necessary to keep the coating membrane 20 nm thick or less, and uniformly thick. However, the coating membrane of a fluorine-based resin is formed by spreading the coating solution, not by vacuum evaporation or sputtering. Spreading the solution to have a membrane whose thickness is controlled at 20 nm or less over the entire lens surface is difficult for a mass production process, or needs an unrealistic system or process to achieve the well-controlled membrane thickness.
As discussed above, it is not easy to coat an anti-reflective membrane of magnesium fluoride with a liquid repellent layer without increasing reflectivity and electrical resistance (i.e., without increasing static electrification) of the anti-reflective membrane.
For forming a liquid repellent layer, Patent Document 1 discloses a method for preventing deposition of stain on a CRT surface by coating the surface with a mist-proof membrane of liquid repellent agent with a perfluoropolyether chain responsible for liquid repellency and alkoxysilane residue at the molecule terminal as a group via which the membrane is bound to another material. Patent Document 2 discloses a technique to improve combustion characteristics of an engine burning fuel directly injected into the cylinder, where the fuel injection valve is treated to be liquid repellent around the nozzle. Desired combustion characteristics may not be secured when deposit produced by the combustion in the cylinder sticks to the valve around the nozzle, because of changed quantity of the fuel injected and fuel injection angle. The technique is intended to prevent the deposit from sticking to the valve, and thereby to control injected fuel quantity and injection angle changes by providing a liquid repellent layer around the nozzle.    Patent Document 1: JP-A-9-255919    Patent Document 2: JP-A-11-311168